This invention pertains to a method and apparatus for logging borehole parameters during a drilling operation and more particularly to a borehole logging system for detecting downhole parameters relating to borehole fluid density.
Drilling fluid or "mud" as it is commonly called may vary in density or "mud weight" for a number of reasons. Such variations can result from changes in the quantity and density of cuttings (particles of formation); changes in the "mud program" at the surface, changes in temperature, etc. Variations in mud density also occur when gas or liquid enter the borehole from the formation. Such influx of formation fluids may likely be the result of formation surpressures or abnormally high pressures. For many reasons, the detection of such abnormal pressures is desirable. An in-depth discussion of this topic is found in a book entitled Abnormal Formation Pressures by Fertl, 1976, Elsevier Scientific Publishing Co. In any event, the detection and quantitative evaluation of overpressured formations is critical to exploration, drilling, and production operations for hydrocarbon resources. World-wide experience indicates a significant correlation between the presence and magnitude of formation pressures, and the shale/sand ratio of sedimentary sections of earth formations. Distribution of oil and gas is related to regional and local subsurface pressure and temperature environments. Knowledge of expected pore pressure and fracture gradients is the basis for efficiently drilling wells with correct mud weights, proper completions and killing of a well without excessive formation damage. Pressure detection concepts are especially important in balanced pressure drilling. Not only does the drilling rate decrease with a high overbalance of mud pressure versus formation pressure, but also lost circulation and differential pressure sticking of the drill pipe can readily occur. On the other hand an underbalance of mud pressure versus formation pressure can cause a pressure "kick." A well may kick without forewarning. Balanced drilling techniques often require only a fine margin between effective pressure control and a threatened blowout.
Present techniques for predicting or detecting abnormal formation pressure include drilling parameters such as drilling rate, torque and drag; drilling mud parameters such as mud gas cuttings, flow line mud weight, pressure kicks, flow line temperature, pit level and pit volume, mud flow rate; shale cutting parameters such as bulk density, shale factor, volume and size of cuttings; well logging data such as conductivity and resistivity surveys, downhole gravity data, nuclear magnetic resonance, and acoustic logs; and direct pressure measuring devices such as pressure bombs, drill-stem test data, and wire-line formation tests. Some comments on the above detection and prediction techniques are as follows: In monitoring flow line mud weight, frequent checks for weight reduction is an indicator for gas cutting and possible overpressures. Continuous mud weight recorders are commercially available to detect and record mud weight at the surface. Several drilling performance indicators can be deduced if mud density and flow rate in and out of the well and stand pipe pressure are accurately known. Formation pressure and salinity are associated and therefore equipment has been developed to measure the gain or loss of chlorides between inlet and flow lines in the mud stream to relate variations in drilling and pressure conditions. Also many modern drilling rig data units record inlet and flow-line mud resistivity. Variations in the total mud volume can be monitored by pit level indicators. Such devices register any large mud-volume reduction, as caused by lost circulation, and monitor large amounts of fluid entry into the borehole, e.g. from unexpected high formation pressures. A first indication of a kick while going into the hole is the observation that the pit level increases in excess of the mud displacement by the pipe run into the hole. Any abnormal rise in pit level caused by mud flow from the annulus will also be reflected in an increasing flow rate, which can be measured by a standard flowmeter. The investigation of shale cuttings is determinative of entry into overpressured environments. An increase in penetration rate, e.g. will result in increased volume of cuttings over the shale shaker. In transition zones, the shape of cuttings is angular and sharp rather than rounded as in normal pressure environments. Of the logging techniques available, the acoustic or sonic log is probably the best log for quantitative pressure evaluation since it is relatively unaffected by changes in hole size, formation temperature, and formation water salinity.
The principle drawbacks of the above techniques have to do with the delay in obtaining such information relative to the potential immediate problems associated with abnormal pressures. For example, a sudden influx of formation fluids such as gas may indicate a potential blowout. Of the above techniques, those obtained "while drilling" are likely to be most helpful to prevent such a blowout from occurring. Such "while drilling" techniques would include drilling parameters, drilling mud parameters, and shale cutting parameters. The drilling parameters such as rate, drag, torque, etc., while occurring in "real time", are not sufficiently definitive to accurately predict the occurrence of a gas kick, for example. Drilling mud and shale cutting parameters are detected at the surface from the circulating mud stream and are delayed with respect to their real time occurrence by the time required for mud return from the bottom of the well bore.
Other detection techniques such as well logging and pressure measuring devices mentioned above are performed after drilling or during the cessation of the drilling operation. With the advent of deeper drilling and increased drilling activity offshore and in hostile surface environments, the costs of drilling have escalated substantially. Therefore, any operation which requires the cessation of drilling in order to be performed, such as lowering a wireline into the borehole, is done at great expense. With the current trend toward many wells being drilled to great depths, formation pressures as related to blowout potential is an increasing factor to be considered. Therefore, a greatly expanded need has arisen to obtain real time data relating to formation and drilling parameters. As a result much effort recently has been expended toward the development of telemetry systems to facilitate the use of measuring while drilling "MWD" techniques. Some of these telemetry systems are now becoming commercially available and as a result the need has arisen to develop downhole detection systems to obtain drilling and formation data to solve the problems enumerated above. The detection of abnormal formation pressures on a real time basis is a priority development in this respect. A basic indicator of abnormal formation pressure is the influx of hydrocarbon fluids or water into the borehole.
It is therefore an object of the present invention to provide a new and improved, simple, reliable and rugged method and apparatus for detecting, downhole, the influx of formation fluids into a borehole during a drilling operation.